There are several commercial needs for more efficient high temperature gas processing whether it is for converting natural gas to liquid fuels or produce low cost hydrogen from hydrocarbon fuels, or produce low cost syngas from a flexible feedstock including natural gas, steam, hydrocarbons, including heavy hydrocarbons such as diesel and jet fuel, as well as cellulose, lignin, coal or their derivatives.
Converting natural gas into synthetic liquid fuels is used in the general field called “reforming”, where commercialization of this process is hampered by the high cost of producing “syngas”, a mixture of CO+H2. In this case, the reaction of methane and steam to produce CO and H2 is highly endothermic and requires significant amounts of heat input. An alternative route is to inject pure oxygen so that the partial oxidation of methane can provide the needed thermal energy and balances the steam reforming process, this is commonly called PDX. However, this solution requires the high capital cost associated with separating oxygen from air.
A less costly route, commonly used by fuel cell developers, generally called autothermal reforming, is to co-feed air directly with the natural gas which initiates the PDX reaction and the production of syngas, the remainder of the O2 required comes through the electrolyte membrane, generating electricity. This achieves the heat generation but causes a problem by adding large amounts of nitrogen that must be heated; this causes problems downstream in that it limits the option to recycle the fuel cell exhaust, which contains up to 20% unspent fuel, but mixed with a significant amount of nitrogen.
An alternate route for fuel processing is to use oxygen conducting ceramic membranes to separate pure oxygen from air and simultaneously react it with the methane and/or other feedstock such as hydrocarbons, polymers such as cellulose, lignin, coal or their derivatives and steam to produce CO and H2. This balances the exothermic reaction and the endothermic reaction, and has the advantage of not injecting the non required nitrogen. The syngas produced can then be converted into liquid fuels during downstream gas processing. This novel process however is limited by reliability of the ceramic membranes. During their use, one side of the ceramic membrane is exposed to air while the other side is exposed to a reducing gas mixture including methane, steam, carbon monoxide and hydrogen. As a result, there is a significant oxygen gradient across the membrane.
These oxygen separating oxide ceramics include either single phase or dual phase materials consist at least one of the following: mixed conducting membranes, belonging to the perovskite family of oxide ceramics or oxides of fluorite family. Under low oxygen partial pressure encountered in suitable feedstock, these ceramics undergo expansion or contraction due to variations in oxygen vacancy concentration, which is dependent on the gradient in partial pressure of oxygen in the environment to which these membranes are subjected.
During operation, steep gradients in oxygen vacancy concentration often cause unacceptably high stresses. The value of tensile stresses is sufficiently high compared to tensile strength that the mechanical durability of these membranes is compromised and they undergo either catastrophic failure or time dependent progressive strength degradation.
Therefore, in order to be commercially viable, there is a need to develop oxygen conducting ceramic membranes that are substantially insensitive to variation in oxygen partial pressure of the feed gases. Alternatively the value of partial pressure of oxygen at the fuel-membrane interface has to be kept sufficiently high so that the stresses are kept to a low value compared to the strength of the membrane. However, such an increase in the partial pressure of oxygen at the interface reduces the driving force needed to achieve high oxygen flux. To compensate for this reduction, membrane thickness has to be reduced making it more susceptible to mechanical failure.